It is well known in the semiconductor manufacturing art that the extremely tiny semiconductor components (hereinafter referred to as “chips”) are generally made up of a base substrate material, such as silicon. Disposed on top of that base are layers of insulation, such as silicon dioxide, and conductive materials, such as metals, which form the complex circuitry of the chip. If one were to examine a delaminated or cross sectioned modem semiconductor chip, which would necessarily require many degrees of magnification, one would see a complex pattern of circuit devices, such as transistors, and multi-leveled, crisscrossing wiring that connects the circuit devices together. For may years, one such metal which was typically used in connection with the manufacture of semiconductors was aluminum. In the last several years, however, copper has become a viable substitute for aluminum in this application.
As is known to those skilled in the art, the actual manufacture of a semiconductor chip is a complex and time consuming process, involving many steps which are dependent upon the usage of tools, materials, and semiconductor masks. Accordingly, once a given chip is manufactured, if the chip proves to be defective, due, for example, to a given wire on the chip making an incorrect contact, it is not easy or expedient to simply create a revised set of masks, and manufacture a new batch of chips. Instead, significant delays may be incurred between the time the wiring problem is detected and actual manufacture of a new chip with correct wiring. Any such delay can have a significant negative effect on the launching of a commercial computer product that is dependent upon the chip because other designers involved with the launch of the complete product may be delayed in doing their work on the project. For example, there may be a staff of computer software programmers waiting to have access to the chip so that they can create software that will run on the chip. Hence, delays in being able to access an actual chip may impact their schedule, which can then have a ripple effect on other stages of the project, too.
To address this overall problem, it is known in the semiconductor manufacturing art to repair a defective chip after it is manufactured. For example, see U.S. Pat. Nos. 5,171,709 and 5,182,230 both to Donelon et al., generally describing the processes for repairing integrated circuits. Through the repair process individual wires inside the chip may be exposed, cut, rerouted, and sometimes reconnected to other wires. Due to the extremely tiny size of these individual wires, the repair process requires considerable precision and time. Thus, the repairing of individual chips is generally not a very cost effective approach to fixing problems with a commercial quantity of chips, but the repair process is suitable for fixing problems with a much smaller number of defective chips, like the number that may be manufactured as prototypes. Importantly, the repair of a relatively small number of prototype chips is a process that addresses the issue discussed above, namely it enables the defective chip to be fixed well enough so that it can be made available for further viability testing and used by other members of a system design team, such as the software programmers.
The repair process generally starts with the determination of what part of the chip needs to be reworked. For example, it may be determined that a given wiring connection should not be made, in which case the objective of the repair would be to cut and terminate the given conductive line on the chip. Or, it may be determined that a given wiring connection should be re-routed to make contact with a point to which it was not originally connected, in which case the objective of the repair would be to cut the given conductive line and reconnect it to the different point. Hence, in many rework processes, the insulative materials of the chip must be selectively removed to expose the conductive wires that need to be cut. Then, the wires themselves need to be selectively cut, too. In order to do the selective removal of the insulative material and the conductive wire, the chip is generally placed in a chamber, into which various gases may be pumped. With the introduction of the right gases, a focused ion beam (FIB) is then used to precisely cut and remove the materials that need to be removed in order to accomplish the repair. Thus, it is the combination of the chemical properties of the gases with the materials of the chip, as made reactive by the ion beam, that all serve to accomplish the selective cutting and removal of materials.
As the semiconductor industry transitioned from aluminum conductive wiring to copper conductive wiring on chips, certain problems developed when attempting to repair chips containing copper. Specifically, gases that had been used successfully in connection with the removal of aluminum conductive lines were proven to be unsuccessful in connection with the removal of copper conductive lines. For example, certain gases customarily used with aluminum were found to spontaneously react with copper, by which is meant that they were not made to react only when excited by an ion beam, but instead they reacted with the copper even without ion beam excitation. Consequently, the use of such gases make it impossible to precisely control the copper removal process, which is essential. Still other gases typically used with aluminum have a tendency to cause copper to evaporate, but then deposit on the sidewalls of the hole that exposed the copper line. This is not desirable since it may create electrical contacts and shorts where they are not intended. Instead it is preferred that any copper that is cut by the ion beam form a reactive specie with the gas in the chamber so that it can be pumped out of the chamber. Yet another problem with known approaches is that certain gases which might work well with copper may have a tendency to attack and damage the internal components of the chamber, which also proved unworkable.
In view of the foregoing, what is needed is a method for cutting copper wires in a semiconductor chip, and in particular a method which is precise and which does not damage the chamber in which the cutting process takes place.